Wordpress - Change Custom Post Type slug

The register_post_type_args filter can be used to modify post type arguments:

add_filter( 'register_post_type_args', 'wpse247328_register_post_type_args', 10, 2 );
function wpse247328_register_post_type_args( $args, $post_type ) {

    if ( 'portfolio' === $post_type ) {
        $args['rewrite']['slug'] = 'stories';
    }

    return $args;
}

@dave-romsey answer didn't work for me, PHP kept telling me Warning: Cannot use a scalar value as an array in /path/to/functions.php So I went the array_merge way.

Complete function you need to add to your child's theme functions.php file:

function update_portfolios_slug( $args, $post_type ) {

    if ( 'portfolios' === $post_type ) {

        //$args['rewrite']['slug'] = 'presidentes';

        $my_args = array(
            'rewrite' => array( 'slug' => 'presidentes', 'with_front' => false )
        );

        return array_merge( $args, $my_args );
    }

    return $args;
}

add_filter( 'register_post_type_args', 'update_portfolios_slug', 10, 2 );

After uploading your code don't forget to "refresh" your Permalinks by going to Settings > Permalinks and hitting Save changes.

Cheers.